The present invention is directed to bicycles and, more particularly, to a bicycle control apparatus that sets a bicycle transmission to a predetermined or desired gear ratio in response to certain conditions.
Bicycles often are equipped with transmissions that provide a plurality of gear ratios to facilitate riding over various terrain. Such transmissions typically comprise either internal or external transmissions. Internal transmissions usually comprise a planetary gear mechanism disposed within the rear wheel hub. External transmissions typically comprise a plurality of sprockets that rotate coaxially with the rear wheel, and a derailleur that switches the drive chain among the plurality of sprockets. In either case, the transmission usually is connected to a shift control lever mounted to the handlebar or to some other convenient location. The rider then may select a desired gear ratio by manually manipulating the shift control lever.
In many instances, the shift control lever is positioned close to the brake lever, and it is sometimes necessary to simultaneously operate the brake lever and the shift lever when decelerating. This makes the shift operation difficult to perform. Electrically powered bicycle transmissions have been developed, and such systems avoid the problems that occur when the shift control lever is positioned close to the brake lever. Systems such as that disclosed in Japanese Published Patent Application No. 2003-011879 use an electrically powered operating unit, such as a motor or solenoid, to operate the bicycle transmission. The operating unit may directly operate the bicycle transmission, or the operating unit may indirectly operate the bicycle transmission via a shift cable. A shift control device that operates in either an automatic or a manual mode controls the operating unit. In manual mode, signals from a manually operated device are used to control the operating unit. In automatic mode, signals for controlling the operating unit are generated automatically based on a riding condition of the bicycle (such as bicycle velocity or crank rotation speed).
Both the operating unit and the shift control device operate using electric power supplied by a power supply. Unfortunately, when the voltage of the power supply has dropped to the point that the operating unit no longer operates reliably, as may occur with frequent shift operations, the system is unable to shift the bicycle transmission, which results in great discomfort to the rider. For example, if shifting is no longer possible while the bicycle transmission is in a low gear, then the rider must pedal at a high rate in order to maintain normal velocity. On the other hand, if shifting is no longer possible while the bicycle transmission is in a high gear, then great pedaling force is required in order to maintain normal velocity.